A typical electric power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric material that include a first semiconducting shield layer, an insulating layer, a second semiconducting shield layer, a metallic tape or wire shield, and a jacket. Surrounding the conductor or core can be accomplished, for example, by extrusion, coating, or wrapping.
These insulated cables are known to suffer from shortened life when installed in an environment where the insulation is exposed to water, e.g., underground or locations of high humidity. The shortened life has been attributed to the formation of water trees, which occur when an organic polymeric material is subjected to an electrical field over a long period of time in the presence of water in liquid or vapor form. The net result is a reduction in the dielectric strength of the insulation.
Many solutions have been proposed for increasing the resistance of organic insulating materials to degradation by water treeing. The most recent solutions involve the addition of polyethylene glycol, as a water tree growth inhibitor, to a heterogeneous low density polyethylene such as described in U.S. Pat. Nos. 4,305,849; 4,612,139; and 4,812, 505. Another solution is the use of a homogeneous polyethylene per se as the organic insulating material, i.e., without the addition of a water tree growth inhibitor. See U.S. Pat. No. 5,246,783. Both of these solutions appear to be steps in the right direction, but there is a continuous industrial demand for improvement partially because power cable is increasingly exposed to harsher environments, and partially because consumers are more concerned with cable longevity, e.g., a service life of 30 to 40 years.